In recent years, computer systems are playing an extensive, advanced, and important role in various types of social infrastructure. A higher performance is required for the computer systems to play the extensive and advanced role. In addition, higher reliability and higher availability are required for the computer systems to play the important role.
In order to meet the higher performance request described above, performance-improving methods for storage systems in which data is stored on a hard disk and the frequently-accessed data is temporarily stored in a volatile memory, such as a DRAM which is a storage medium higher in speed than the hard disk, have been adopted. However, the volatile memory such as the DRAM loses the data held therein when electric power supply is interrupted. In order to prevent this situation, several tiers of backups are disposed in centers that activate the storage systems in general so that the electric power supplied to the storage systems is not interrupted despite a power outage and the like. Electrical powering of the storage systems may still be interrupted nonetheless. In the case of small-scale operation, an electric power backup may not be disposed as the case may be. Given that the information stored in the storage system is important information relating to the social infrastructure and the information is not allowed to be lost in any case, the data on the volatile memory such as the DRAM must not be lost even in a case where the electrical powering of the storage system is interrupted. In the meantime, methods are in use for saving the data stored in the volatile memory such as the DRAM to a non-volatile memory such as a flash memory by using electric power of a battery mounted on the storage system in the event of an electric power supply failure (PTL 1). In this case, the reliability of the storage system can be ensured.